Environ. Sci. Technol. 2007, 41, 5224-5230
Metal Emissions from Brake Linings and Tires: Case Studies of Stockholm, Sweden 1995/1998 and 2005 DAVID S. T. HJORTENKRANS,* B O G . B E R G B A¨ C K , A N D A G N E T A V . H A¨ G G E R U D School of Pure and Applied Natural Sciences, University of Kalmar, Sweden
Road traffic has been highlighted as a major source of metal emissions in urban areas. Brake linings and tires are known emission sources of particulate matter to air; the aim of the current study was to follow the development of metal emissions from these sources over the period 1995/ 1998-2005, and to compare the emitted metal quantities to other metal emission sources. Stockholm, Sweden was chosen as a study site. The calculations were based on material metal concentrations, traffic volume, particle emission factors, and vehicle sales figures. The results for metal emissions from brake linings/tire tread rubber in 2005 were as follows: Cd 0.061/0.47 kg/year, Cu 3800/5.3 kg/year, Pb 35/3.7 kg/year, Sb 710/0.54 kg/year, and Zn 1000/4200 kg/ year. The calculated Cu and Zn emissions from brake linings were unchanged in 2005 compared to 1998, indicating that brake linings still remain one of the main emission sources for these metals. Further, brake linings are a source of antimony. In contrast, Pb and Cd emissions have decreased to one tenth compared to 1998. The results also showed that tires still are one of the main sources of Zn and Cd emissions in the city.
Introduction Globally, metals are accumulated in the technosphere, especially in urban areas where the in-use stock is increasing, e.g., for copper (1) and zinc (2). From the stock, metal emissions to the biosphere may be significant, e.g., refs 3 and 4. Several studies have stressed road traffic as a source of metal emissions, particularly in urban areas (e.g., refs 5 and 6), and brake linings as well as tires are known emission sources of particulate matter to the surrounding environment (7-10). Asbestos was used in brake linings until the 1980s when it was banned in Sweden to improve both working environments and the outdoor air quality of cities. The replacement material was a complex mixture of various substances: reinforcement fibers of glass, steel, and plastic; “friction modifiers”; fillers in the form of antimony compounds and brass chips; and iron filings and steel wool as heat-conducting materials (11). The materials used in brake linings are of environmental relevance as a greater part of the material is dispersed directly into the environment when used. Westerlund (12) measured metal concentrations in brake linings * Corresponding author phone: +46-480-446227; fax: +46-480447305; e-mail:
[email protected]. 5224
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and calculated the metal emissions in Stockholm for 1998 for a number of metals including cadmium, copper, lead, and zinc. It has clearly been shown that brake linings are a major source of metal emissions in urban areas (e.g., refs 13 and 14). As there are only a few similar studies, EMEP (the Co-operative Program for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe) has based its calculation model for brake lining emissions mainly on data from the Westerlund study(14). With an increasing willingness to phase out a number of metals in the past decade, as well as the introduction of “lead-free” brake linings (11), there is a need to update Westerlund’s study to enable the changes in material and emissions to be monitored. Further, studies have shown that large amounts of antimony might be emitted from brake linings, as antimony (Sb2S3) is used by some manufacturers as a filler and lubricant in brake linings (e.g., refs 5, 15-17). Further, Uxeku ¨ ll et al. (17) emphasize that Sb in friction material might pose a human cancer risk and its use should, therefore, be discouraged. The total Zn emissions from tires in Sweden have been calculated at 150 tonnes/year, which is a significant figure when compared to other sources (18). The metal emissions in Stockholm have been briefly quantified earlier (1995) (19). Furthermore, So¨rme and Lagerkvist (20) calculated metal emissions from tires via stormwater to the drainage area for one of Stockholm’s wastewater treatment plants. The calculated emissions in their study are based on the metal concentrations in tires given by Legrett and Pagotto (21). This is one of the few studies to analyze metal concentrations in the part of the tire that wears while driving. Most other studies have analyzed the total concentration in tire chips, as they focused on emissions from tires as a combustion fuel. This method might result in overestimation as the tire body often contains metal reinforcements. An updated calculation of Cd emissions from tires in Stockholm can be found in Hjortenkrans (22), who concludes that there is a lack of relevant metal analysis studies of tire tread rubber. According to the directive of the European Parliament and Council (23), materials and components in vehicles produced after July 2003 should not contain lead, mercury, cadmium, or hexavalent chromium. Brake linings were one of the components added in June 2002 to Appendix II of the directive as an exception to these restrictions (24). This appendix states that the use of copper containing more than 0.5 weight percentage lead in brake linings is allowed for vehicle models approved before July 2003, including maintenance of these vehicles until July 2004. After that time, a concentration of up to 0.4 weight percentage lead in copper in brake linings is permissible until July 2007, provided that it is not intentionally added. In Sweden, the directive has only been implemented for pick-up trucks and private cars that are not approved EU models (25). In practice this means that Swedish EU-approved models of private cars are allowed to use replacement brake linings containing lead. The aim of the current study was to follow the development of metal emissions from brake linings and tire tread rubber over the period 1995/1998-2005, and to put the emitted metal quantities in relation to other metal emission sources in an urban area. The focus was on Cd, Cu, Pb, Sb, and Zn, and for tires, chromium and nickel. Stockholm, the capital of Sweden, was chosen as a study site. The city is relatively densely populated with approximately 700 000 inhabitants in an area of 190 km2. 10.1021/es070198o CCC: $37.00
2007 American Chemical Society Published on Web 06/22/2007
Materials and Methods Sampling. Westerlund’s (12) study of metal emissions from brake lining wear was based on a manual vehicle count, with brake linings of the most common 63% of vehicles analyzed for metals. As vehicle counts are rare, the current study uses vehicle sales figures for 2004 (Supporting Information Table S1), reflecting the vehicle pool that will contribute to metal emissions from brake linings for several years to come. In accordance with Westerlund, brake linings from the two most common lorry and bus models were also examined. Some of the brake linings were of the same kind for different models. A total of 42 vehicle replacement brake linings were sampled. As not all owners of private cars use branded replacement brake linings, brake lining samples from independent suppliers were needed. The independent sector has seen a large increase in market share since new EU regulations were introduced in 2003 (26). The new regulations allow independent garages to service and repair new cars, as long as the vehicle manufacturer’s requirements are met. Accordingly, the proportion of repairs carried out by independent garages is expected to continue to increase. Today, the independent sector represents approximately half of the aftersales market for spare parts. To analyze the metal concentrations in replacement brake linings for private cars from independent suppliers, the two largest suppliers in Sweden were selected, namely Mekonomen and Meca (26). Based on brake linings sales figures (27-29), five car models were chosen and brake linings for front and rear wheels sampled. A total of 20 brake linings from independent suppliers were analyzed. Titanium-covered drills where used to obtain samples from the brake linings, as titanium was not included in the analysis. The composition of tire rubber varies by year of manufacture, make and model, factory and rubber mass batch. For economic, practical, and environmental reasons, discarded tires were sampled. The aim was to take tire tread rubber samples from manufacturers corresponding to 75% of the total annual turnover of tires in 2005 (Supporting Information Table S2). Make, model, and year were noted, and a total of 52 tires were sampled. Rubber samples from the outer 5 mm of the tread were removed with a knife. The rubber chips were fragmented with scissors and then washed repeatedly in 18.2 MΩ/cm2 Milli-Q water. Analytical Methods. Samples were dried to constant weight at 60 °C. About 0.4000 g of sample was digested in 3 mL concentrated HNO3 and 3 mL concentrated HCl in closed vessels in a microwave oven (Perkin-Elmer). The program used was 400 W for 6 min, 900 W for 10 min, and cooling for 15 min. The vessels were not opened until the temperature of the solution had decreased to below 30 °C to avoid evaporation of volatile metal compounds (such as antimony chlorides). The samples were finally diluted to 100 mL with 18.2 MΩ/cm2 Milli-Q water. The metal concentrations were analyzed using an atomic absorption spectrophotometer (AAS) and a graphite furnace atomic absorption spectrophotometer (GFAAS) (Perkin-Elmer Aanalyst 800). Details of the instrumental parameters are given in Tables S3 S4 in the Supporting Information. Variation and Quality Control. To analyze the variation of metal concentrations within one unit, six replicates of two brake linings and two to six replicates from 10 different tires were made. To control the precision of the analysis method, six brake lining and six tire tread samples were sent to an external accredited laboratory for control (Analytica, Luleå Sweden). Analytica is not accredited for metal analyses of brake lining and tire rubber composition; instead they used two different accredited digestion methods for plastic. Those methods differed slightly from those used in our study, as aqua regia (1:3) was used for brake linings and concentrated
HNO3 and H2O2 for tires. Both digestion methods used closed vessels in a microwave oven. Analytica’s results were compared with our analyses using a one-sample t test for all metals. The precision, accuracy, and reproducibility were checked with control blanks and quality control solutions (SPS-SW1, SPS-WW1, Spectrapure Standards AS, Oslo, Norway and Certified Reference Material TM-15, Environment Canada, National Water Research Institute, Burlington, Ontario, Canada). Calulation of Metal Emissions. The amount of particulate matter from brake linings was calculated differently depending on the type of vehicle. Calculations were based on the wear of brake linings before replacement, the weight of the brake linings, the distance driven before replacement, and the total distance driven per year for each vehicle type (for details see Westerlund, ref 12). The amount of particulate matter from each axle was corrected for updated traffic volumes. The assumption that brake linings from branded and independed suppliers have the same lifetime has been made. Westerlund assigned 40% of traffic volume to new vehicles (less than 4 years old) and 60% to older vehicles, making the assumption that all new vehicles were using branded brake linings and all old vehicles were using unbranded brake linings. For the sake of comparability, the same assumptions were made in this study. The calculations of metal emissions from brake linings were made according to eq 1.
ME )
∑ v
[
]
(Pfront × Cfront) + (Prear × Crear) 1000
(1)
where ME is the amount of metal emitted per year (kg/year), v is the type and make of vehicle (type: private car, pick-up, bus, or lorry), P is the the total amount particulate matter emitted from brake linings from each axle per year (tonnes/ year) and includes particulate matter emitted per kilometer and distance driven by a specific type and make of vehicle, and C is the mean metal concentration in brake linings for each axle (mg/kg). The tire emission calculations were based on traffic volume for Stockholm and Sweden, respectively (Supporting Information Table S5), sale figures for tires (where the annual percentages sales of each brand are assumed to reflect the distance driven with that brand), particle emission factors for tire wear, and analyzed metal concentrations in tires. The calculations of metal emissions from tires were made according to eq 2.
ME )
∑ v
[
]
P × C × TV × F 1000
(2)
where TV is traffic volume (Mvkm/year) and F is the proportion driven with each type of vehicle and brand, expressed as annual sales figures. A more straightforward calculation excluding sales figures was also performed. Particle emission factors for tires have been compiled by the EEA (14) and Gustavsson (30). The factors vary by 1 order of magnitude depending on which references are used. The emission factors for private cars range from 24 to 360 mg/ vkm (vehicle kilometers) (mean ( standard deviation 122 ( 96 mg/vkm), for pick-ups 53-112 mg/vkm (102 ( 16 mg/ vkm), and for heavy vehicles 136-1403 mg/vkm (628 ( 450 mg/vkm). A mean metal concentration of a brand regardless of model has been used in the calculations. Calculations for pick-ups follow the market shares for private cars. As no newer statistics for heavy vehicles were known to us, we followed the Swedish market share data of Ahlbom and Duus (9) (Supporting Information Table S6). VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. 1. Mean metal concentrations (Cd, Cu, Pb, Sb, and Zn) in private car brake linings from branded and independent suppliers in 2005 in Sweden (mg/kg) (as antimony and copper concentrations were low in brake linings from independent suppliers, the columns are not shown). Estimates of traffic volumes nationally are made every 4 years. The most recent calculations are from 2002. However, traffic volume has increased at a rate of 3.5% from 2002 to 2005, and thus the values used here are figures for 2002 adjusted by 3.5% (31). For Stockholm the increase for the period 2002-2005 is estimated to be 2.5% (32). Traffic volumes were divided according to type of vehicle, as shown in Table S5 in the Supporting Information. The calculated emissions for Stockholm are presented as means ( relative standard deviation (rsd) as a percentage, together with an interval with minimum and maximum values based on the relative standard deviation. Calulations of Uncertainties. As the calculations are multiplicative expressions, the uncertainties have been calculated according to eq 3.
r.s.d. )
σME ) ME
x( ) ( ) ( ) ( ) σP P
2
+
σC C
2
+
σTV TV
2
+
σF F
2
(3)
where r. s. d. is the relative standard deviation (%), σME is the standard deviation for amount of metal emitted per year, ME is the amount of metal emitted per year, σP is the standard deviation for emitted amount of brake lining or tire compound, P is the mean for emitted amount of brake lining or tire compound, σC is the standard deviation for metal concentration in brake lining or tire compound, C is the mean for metal concentration in brake lining or tire compound, σTV is the standard deviation for traffic volume, TV is the mean for traffic volume, σF is the standard deviation share of traffic volume, expressed as annual sales figures, for tires only, and F is the mean for share of traffic volume, expressed as annual sales figures, for tires only
Results and Discussion Metal Concentrations. The mean metal concentrations in analyzed branded brake linings for private cars from 2005 were for front/rear brake linings 1.2/4.0 ppm Cd, 130 000/ 130 000 ppm Cu, 120/2900 ppm Pb, 23 000/9500 ppm Sb, and 27 000/37 000 ppm Zn (Figure 1, for details see Supporting Information Table S7). Here, Cu and Zn concentra5226
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tions are high in both front and rear brake linings and of the same magnitude as in 1998. Antimony also shows a high concentration, especially in front brake linings (antimony was not analyzed in 1998). The mean Pb concentration in branded brake linings from 2005 is much lower than the corresponding values from 1998. In 1998 the Cd concentrations were between
